Remove beast::Thread (RIPD-1189):

All uses of beast::Thread were previously removed from the code
base, so beast::Thread is removed.  One piece of beast::Thread
needed to be preserved: the ability to set the current thread's
name.  So there's now a beast::CurrentThreadName that allows the
current thread's name to be set and returned.

Thread naming is also cleaned up a bit.  ThreadName.h and .cpp
are removed since beast::CurrentThreadName does a better job.
ThreadEntry is also removed, but its terminateHandler() is
preserved in TerminateHandler.cpp.  The revised terminateHandler()
uses beast::CurrentThreadName to recover the name of the running
thread.

Finally, the NO_LOG_UNHANDLED_EXCEPTIONS #define is removed since
it was discovered that the MacOS debugger preserves the stack
of the original throw even if the terminateHandler() rethrows.

src/ripple/core/impl/SNTPClock.cpp

@@ -20,10 +20,8 @@
 #include <BeastConfig.h>
 #include <ripple/core/impl/SNTPClock.h>
 #include <ripple/basics/Log.h>
-#include <ripple/basics/ThreadName.h>
 #include <ripple/basics/random.h>
-#include <ripple/beast/core/Thread.h>
-#include <ripple/core/ThreadEntry.h>
+#include <ripple/beast/core/CurrentThreadName.h>
 #include <beast/core/placeholders.hpp>
 #include <boost/asio.hpp>
 #include <boost/optional.hpp>
@@ -203,13 +201,8 @@ public:
 
     void doRun ()
     {
-        setCallingThreadName("SNTPClock");
-
-        // Get the address of an overloaded asio method
-        using Pio_service_mem = std::size_t (boost::asio::io_service::*)();
-        Pio_service_mem pRun = &boost::asio::io_service::run;
-
-        threadEntry (&io_service_, pRun, "SNTPClientImp::doRun()");
+        beast::setCurrentThreadName("rippled: SNTPClock");
+        io_service_.run();
     }
 
     void